Why should you never add hydrated lime to ASTM C91 masonry cement?

ASTM C91 masonry cement already contains factory-interground Portland or hydraulic cement, limestone plasticizers, and air-entraining additives. Adding separate hydrated lime alters the binder-to-aggregate ratio, lowers 28-day compressive strength below ASTM C270 minimums, and produces excessive shrinkage cracking.

How many bags of masonry cement are needed per 100 cinder blocks?

Laying 100 standard 8×8×16-inch concrete masonry units with 3/8-inch mortar joints requires approximately 1.3 to 1.5 bags (70-lb Type S or 75-lb Type M), 0.15 cubic yards (approx. 400 lbs or 0.2 tons) of ASTM C144 sand, and 6 gallons of water, factoring in standard 10% jobsite waste.

What is the difference between a 70-lb and a 75-lb masonry cement bag?

Under ASTM C91 packaging standards, Type N and Type S masonry cements are packaged in 70-lb bags, whereas heavier Type M masonry cement is packaged in 75-lb bags. Regardless of weight, both yield 1 loose cubic foot of cementitious binder and mix with 3 cubic feet of sand under ASTM C270 volumetric proportioning.

When is Type S masonry cement required instead of Type N or Type M?

Type S masonry cement (1,800 psi minimum compressive strength) is mandatory under IBC and IRC codes for reinforced structural masonry, foundation walls, retaining walls, and any CMU below or at grade subject to lateral earth or wind loads. Type N (750 psi) is restricted to non-loadbearing partitions and above-grade veneers, while Type M (2,500 psi) is reserved for heavy civil foundations.

ASTM C91 & ASTM C270 Table 2 Preblended Mortar Standards

Masonry Cement Mortar Calculator

Laying 1,000 standard cinder blocks with ASTM C91 Type S masonry cement requires 13 bags (70 lbs each), 1.44 cubic yards of damp masonry sand, and 59 gallons of water, eliminating the need for separate hydrated lime.

Size 70-lb and 75-lb ASTM C91 preblended masonry cement bags, ASTM C144 masonry sand volumes and tonnages, water batching, and mixer shovel ratios under ASTM C270 Table 2.

ASTM C270 Project Presets (1-Click)

Live Mortar Takeoff

13Bags (70 lbs)• 1.44 yd³ sand (~1.94 tons)

Details

Total CMU Block CountStandard 8×8×16 units

Total standard hollow cinder blocks requiring bed and head joints.

ASTM C270 Mortar Type

Type S is required for reinforced loadbearing masonry and retaining walls.

Bag Packaging Weight

1 bag equals 1 loose cubic foot binder volume under ASTM C270.

ASTM C270 Volumetric Mix Ratio

1 Part Cement : 3 Parts Sand (1:3 Standard)

Table 2 limits sand to 2¼ to 3 times the cement volume (3 cu ft/bag).

Critical Masonry Cement Rule: Never Add Hydrated Lime

ASTM C91 Masonry Cement is already preblended at the factory with plasticizers and air-entraining additives. Adding separate hydrated lime or Portland cement on the jobsite is strictly prohibited as it disrupts air content and lowers compressive strength.

ASTM C91 Mortar Takeoff

ASTM C91 Preblended Masonry Cement

13Bags (70 lbs each)

910 lbs total cementitious binder (Type S)

ASTM C144 Sand

1.44cu yds

~1.94 tons (38.9 cu ft)

Water Demand

59gallons

~4.5 gal / bag batch

Total Mortar Yield

37cu ft

Includes 10% joint waste

1-Bag Mixer Batches

13batches

18 shovels sand per bag

No Hydrated Lime Required

ASTM C91 preblended masonry cement contains factory-interground plasticizers. Only add clean ASTM C144 masonry sand and potable water.

Estimated Materials Cost:~$276

Cement: $189

Sand: $87

ASTM C270 Table 2 Formulation

Proportions: 1 part masonry cement to 3 parts damp loose masonry sand. Mechanical mixer run time: 3 to 5 minutes.

CinderCalc

ASTM C91 Preblended Masonry Cement Mortar Procurement Proposal

cinderblockscalculator.com • ASTM C91 / ASTM C270 Table 2 Proportion Specification

Date: October 30, 2026

Jobsite Material Order

Total Block Count: 1,000 Standard 8×8×16 CMU

Mortar Specification: Type S (1,800 PSI Compressive Strength)

Bag Packaging: 70-lb Bag (70 lbs/bag)

Mix Volumetric Proportion: 1 part cement : 3 parts damp sand

Total Cement Weight: 910 lbs

Total Sand Weight: ~1.94 tons (1.44 cu yds)

Water Demand: ~59 gallons total

Estimated Material Cost: ~$276

Material Item	Takeoff Quantity	Specification & Batching Rule
ASTM C91 Masonry Cement	13 bags	70-lb bags (910 lbs total binder). DO NOT ADD LIME.
ASTM C144 Damp Masonry Sand	1.44 cu yds (~1.94 tons)	Clean damp masonry sand (3.0 cu ft per bag of cement, ~18 shovels/bag).
Potable Mixing Water	~59 gallons	Approx. 4.0 to 5.0 gallons per 1-bag mixer batch to achieve buttery workability.
Total Wet Mortar Yield	37 cu ft	Compacted wet mortar volume including 10% standard jobsite waste.

Standard 1-Bag Mechanical Mixer Batching Protocol:

Add approximately 3/4 of clean mixing water (~3.5 gallons) into the rotating paddle drum.
Add approximately 9 shovels (1.5 cu ft) of ASTM C144 damp masonry sand.
Add 1 full bag of ASTM C91 Masonry Cement (70 lbs).
Add remaining 9 shovels of sand (total 18 shovels = 3.0 cu ft) and balance water.
Mix continuously for 3 to 5 minutes per ASTM C270 Section 5.3. Never add lime.

Disclaimer: This material takeoff is for procurement and planning purposes only. Compressive strength, bond beam details, and grout reinforcement must comply with locally adopted building codes (TMS 402/602, IBC 2024, IRC R606) and the licensed engineer of record.

Generated by CinderCalc • cinderblockscalculator.com

ASTM C270 Table 2 Masonry Cement Proportion & Strength Table

Standard commercial preblended masonry cement specifications under ASTM C91 and ASTM C270 Table 2. Unlike site-blended Portland-lime mortars, masonry cements contain factory-calibrated plasticizers and air-entraining agents, requiring only clean damp sand and water.

Mortar Type	28-Day Strength (ASTM C109)	Cement Volume Part	Sand Volume (ASTM C144)	Bag Packaging (ASTM C91)	Structural Applications
Type M	2,500 PSI (17.2 MPa)	1 Part Type M	2¼ to 3 times cement volume (3 cu ft/bag)	75-lb Bag (nominal 1 cu ft)	Heavy retaining walls, foundation footings, deep earth contact
Type S	1,800 PSI (12.4 MPa)	1 Part Type S	2¼ to 3 times cement volume (3 cu ft/bag)	70-lb Bag (nominal 1 cu ft)	Reinforced structural CMU, below-grade foundations, seismic walls
Type N	750 PSI (5.2 MPa)	1 Part Type N	2¼ to 3 times cement volume (3 cu ft/bag)	70-lb Bag (nominal 1 cu ft)	Above-grade exterior veneers, interior non-loadbearing partitions

Worked Example: Mortar Takeoff for 1,000 Standard 8×8×16 CMU Blocks

To illustrate how contractors and estimators size materials for commercial blockwork, consider a 1,000-block structural foundation run built with standard 8×8×16 hollow CMU, 3/8-inch face-shell mortar joints, and ASTM C91 Type S preblended masonry cement.

1Mortar Yield & Block Coverage

Under standard masonry estimating practices, 100 8-inch CMU units require approximately 3.3 to 3.7 cubic feet of wet compacted mortar. Laying 1,000 units yields a net mortar demand of 33.3 cu ft. Applying a standard 10% allowance for head joint extrusion, bed trimmings, and board waste:

Total Mortar = 33.33 × 1.10 = 36.67 ≈ 37.0 cu ft

2ASTM C91 Cement Bags (70 lbs)

Under ASTM C270 Table 2, 1 bag of masonry cement produces 1 loose cubic foot of cementitious material, which combines with 3 cu ft of sand to yield approx. 3.0 cu ft of mixed mortar. Dividing total mortar demand by bag yield:

Cement Bags = 36.67 ÷ 3.0 = 12.22 → Round up to 13 Bags (910 lbs)

3ASTM C144 Masonry Sand Volume & Tonnage

With 13 bags batching at 3.0 cu ft of damp loose sand per bag, total sand volume equals 39.0 cubic feet. Converting cubic feet to cubic yards and commercial short tons (assuming damp bulk density of ~2,700 lbs/cu yd or 1.35 tons/cu yd):

Sand Volume = 39.0 ÷ 27 = 1.44 cu yds × 1.35 = 1.94 Tons

4Potable Mixing Water & Batching

Each 70-lb bag of Type S masonry cement requires approximately 4.5 gallons of clean, potable water depending on sand dampness and ambient temperature. Total jobsite water requirement:

Water Demand = 13 Bags × 4.5 gal = 58.5 ≈ 59 Gallons

ASTM C91 Preblended Masonry Cement vs. Site-Mixed Portland & Lime

When specifying mortar for concrete masonry unit (CMU) construction, engineers and mason contractors typically choose between preblended ASTM C91 Masonry Cement and traditional ASTM C270 Portland Cement-Lime (P/L) formulations.

Preblended Consistency

ASTM C91 cements combine Portland cement clinker, limestone, and plasticizing agents interground in industrial ball mills. This eliminates jobsite batching errors caused by laborers measuring separate bags of hydrated lime (ASTM C207).

Board Life & Workability

Masonry cements feature higher microscopic air entrainment (typically 12% to 21% air content) than Portland-lime mortars. This gives the wet mortar exceptional board life, buttery spreadability, and enhanced freeze-thaw durability.

Flexural Bond Strength

While masonry cement provides high compressive strength and workability, Portland-lime mortars achieve higher flexural tensile bond strength. For high-seismic zones (SDC D–F), check whether the structural engineer specifies ASTM C1329 Mortar Cement or P/L mortar.

Jobsite Mechanical Mixer Protocol: The Standard 18-Shovel Rule

Under ASTM C270 Section 5.3 and TMS 602 Section 3.2 C, all mortar must be mixed in a mechanical batch mixer with rotating paddles for a minimum of 3 minutes and maximum of 5 minutes after all constituents are added. Follow this precise contractor charging sequence:

Step 1Water Charge

Add ~3.5 gallons (approx. 75% of mix water) to lubricate rotating paddles.

Step 2Half Sand

Add 9 standard shovels (~1.5 cu ft) of damp ASTM C144 masonry sand.

Step 31 Bag Cement

Empty 1 whole bag (70-lb or 75-lb) of ASTM C91 masonry cement.

Step 4Balance Sand

Add remaining 9 shovels of sand (total 18 shovels = 3.0 cu ft) and balance water.

Step 53–5 Min Mix

Mix thoroughly. Discard any unused mortar after 2.5 hours from initial batching.

Jobsite Retempering & Pot Life Limitations

Mortar that stiffens due to evaporation on the mortar board may be retempered with clean water within the first 2 hours of mixing to restore trowel workability. However, under TMS 602 Section 3.2 C.4, never retemper mortar older than 2.5 hours. Once initial cement hydration begins, adding water dissolves developing crystal matrices and drastically weakens compressive bond strength.

Calculation Assumptions & References Planning Reference

Material estimates depend on the selected dimensions, product yields, and waste allowance.

Content date: October 2026

Related references: ASTM C91 (Standard Specification for Masonry Cement), ASTM C270 Table 2 (Proportion Specification for Masonry Cement Mortars), ASTM C144 (Standard Specification for Aggregate for Masonry Mortar), TMS 602 / ACI 530.1 / ASCE 6 (Specification for Masonry Structures), NCMA TEK 09-01A (Mortars for Concrete Masonry). Confirm the applicable edition and local requirements.

A material estimate is not a structural design or code approval.

Masonry Q&A

ASTM C91 Masonry Cement Mortar FAQ

Expert technical answers on preblended masonry cement proportions, ASTM C144 sand ratios, hydrated lime prohibitions, and ASTM C270 compliance.

Under ASTM C270 Table 2 (Proportion Specification for Masonry Cement Mortars), the volumetric ratio is 1 part ASTM C91 masonry cement to 2-1/4 to 3 parts damp, loose ASTM C144 masonry sand. Standard field practice pairs 1 bag (1 loose cubic foot) with 3 cubic feet of damp sand (approx. 18 standard shovels) and 4.5 to 5 gallons of clean water.