Why is hydrated lime required in site-mixed Portland mortar?

Pure Portland cement and sand mortar dries brittle and stiff, leading to rapid water loss into porous dry CMU blocks and poor bond adhesion. Type S hydrated lime acts as a microscopic plasticizer and water-retention agent, giving wet mortar a buttery workability that clings to vertical block head joints and allows autogenous healing of micro-cracks over time.

What type of hydrated lime must be used under ASTM C270?

Masonry specifications strictly require ASTM C207 Type S (Special) hydrated lime. Type S lime has a minimum water retention of 85% and limits unhydrated oxide content to 8%, preventing delayed expansion popping in hardened joints. Agricultural lime, dolomitic quicklime, or Type N hydrated lime cannot be substituted in structural mortar.

How many CMU blocks will one 94-lb sack of Portland cement lay?

In a standard ASTM C270 Type S proportion mix (1 sack Portland cement + 0.5 bag lime + 18 shovels sand), one 94-lb bag yields approximately 3.8 cubic feet of workable wet mortar. With 3/8-inch face-shell mortar bedding on standard 8x8x16 CMU blocks and typical 10% waste, one sack lays approximately 45 to 55 hollow blocks.

What is the difference between ASTM C270 Type M, Type S, Type N, and Type O mortar?

Type M (2,500 psi | 1:0.25:3.75) is formulated for high compressive loads and below-grade foundations. Type S (1,800 psi | 1:0.5:4.5) is the primary structural standard for load-bearing block walls and seismic zones. Type N (750 psi | 1:1:6) provides maximum workability for above-grade exterior veneers and parapets. Type O (350 psi | 1:2:9) is a low-strength, high-lime mix reserved for interior non-bearing walls and historic tuckpointing.

ASTM C270 Volumetric Batching Standards

Portland Lime Mortar Calculator

Under ASTM C270, laying 1,000 standard CMU blocks with Type S mortar requires 8 bags of Type I/II Portland cement (94 lbs), 4 bags of Type S hydrated lime (50 lbs), 1.09 cubic yards of sand, and 44 gallons of water.

Calculate site-mixed 94-lb Portland cement sacks, 50-lb Type S hydrated lime bags, damp masonry sand volume, and water ratios for ASTM C270 Type M, S, N, and O field mortar batches.

Project Scale Presets 1-Click Load

Type S Takeoff

9Portland Bags• 5 Lime • 1.1 yd³ Sand

Details

Total CMU Block Count (8×8×16 nominal)~30 blocks per cu ft wet mortar

ASTM C270 Mortar Specification Type1,800 psi 28-day rating

Industry standard for load-bearing CMU block walls, shear walls, and seismic zones.

Jobsite 1-Sack Mixer Batch RecipePaddle Mixer Standard

Portland Cement1 Sack(94 lbs)

Hydrated Lime0.5 Bag(25 lbs)

Damp Sand20 Shovels(~4.5 cu ft)

Clean Water~5.5 Galto consistency

Each 1-sack Type S batch yields approximately 4.3 cu ft of wet workable mortar, enough to lay approximately 54 standard 8×8×16 CMU blocks with 3/8″ bed and head joints.

Material Order Takeoff

ASTM C270 Type S

Portland Cement (94-lb Bags)ASTM C150 Type I/II

9Sacks to Order

Total weight: 846 lbs (9 cu ft bulk volume)

Hydrated Lime (50-lb Bags)ASTM C207 Type S Hydrated Lime

5 Bags250 lbs

Damp Masonry SandASTM C144 (~134 #2 shovels)

1.1 yd³~1.49 Tons (29.7 cu ft)

Clean Mixing WaterPotable, free of acids/oils

~50 Gal~417 lbs

Total Wet Mortar VolumeIncludes 10% waste & scrapings

~37 cu ft1.37 yd³

Illustrative Material CostPlanning Range

$244 – $336

Assumes $13.50–$18.00/bag Portland, $11.00–$15.50/bag Lime, and $45–$65/ton bulk sand. Labeled illustrative planning example only; request dated local masonry supplier quotes.

CMU estimates use nominal face dimensions and illustrative mortar yields (30 standard 8-inch units per 80-lb premix bag before waste; thickness adjustments apply). Confirm product yields, unit core volumes, supplier pack sizes and local prices before ordering. Reinforcement quantities are material allowances; laps, hooks, embedment and engineered design are excluded.

ASTM C270 Table 1: Mortar Proportions by Volume

Official proportion specification requirements for field-mixed unit masonry mortar. Proportions are calibrated by loose dry volume where 1 sack of Portland cement equals 1.0 cubic foot.

Mortar Type	Portland Cement (ASTM C150)	Hydrated Lime (ASTM C207 Type S)	Damp Masonry Sand (ASTM C144)	Min. Compressive Strength (28-day)	Primary Applications
Type M	1 part	1/4 part (0.25)	2¼ to 3× sum (3.0 – 3.75 parts)	2,500 psi	Below-grade foundations, heavy retaining walls, manholes, high earth pressure
Type S	1 part	1/2 part (0.50)	2¼ to 3× sum (3.38 – 4.5 parts)	1,800 psi	Structural CMU block walls, seismic design, wind-load shear walls, standard CMU
Type N	1 part	1 part (1.00)	2¼ to 3× sum (4.5 – 6.0 parts)	750 psi	Above-grade exterior walls, brick/stone veneer, chimneys, non-load parapets
Type O	1 part	2 parts (2.00)	2¼ to 3× sum (6.75 – 9.0 parts)	350 psi	Interior non-load partitions, historic masonry restoration, soft brick repointing

Note: ASTM C270 establishes that sand volume must measure not less than 2¼ times and not more than 3 times the sum of the separate volumes of cementitious materials.

Jobsite Mechanical Paddle Mixer Loading Sequence

To achieve uniform binder dispersion and prevent unhydrated cement or lime balls from sticking to mixer paddles, follow the standardized 4-step sequence compliant with ASTM C270 Section 5.3:

Initial Water & Cushion Sand

Start the mechanical mixer drum. Pour approximately 75% of total mixing water (~4 gallons for a 1-sack Type S batch) into the drum, immediately followed by half of the damp masonry sand (9 shovels). This lubricates the rubber wiper paddles and drum walls.

Charge Cement & Hydrated Lime

Add the full 94-lb sack of Portland cement and the measured 25-lb half bag of ASTM C207 Type S hydrated lime directly into the swirling water-sand bed. Allow the rotating paddles to blend the materials into a rich, clump-free slurry for 60 seconds.

Add Final Sand & Temper Water

Add the remaining half of the damp masonry sand (9 shovels). Gradually add remaining water in small increments while observing mortar consistency until the mix rolls smoothly off the paddle blades with a rich, buttery spread.

Timed 3-to-5 Minute Mechanical Mix

Mix continuously for a minimum of 3 minutes and a maximum of 5 minutes after all materials are in the drum. Mixing under 3 minutes leaves dry pockets; mixing over 5 minutes entrains excess air voids that compromise 28-day compressive strength.

Key Material Standards: Portland, Lime & Sand

Portland Cement (ASTM C150)

Use standard 94-lb bags of ASTM C150 Type I (normal) or Type II (moderate sulfate resistance / heat of hydration). Blended hydraulic cements conforming to ASTM C595 (such as Type IL Portland-limestone cement) are also broadly code-approved under TMS 602.

Type S Hydrated Lime (ASTM C207)

Only Type S (Special) hydrated lime provides the 85% minimum water retention and immediate plasticity needed for blockwork. Never use agricultural lime or Type N lime, which can cause unhydrated magnesium oxide pops in finished masonry joints.

Masonry Sand (ASTM C144)

Aggregate must be clean, natural or manufactured damp masonry sand conforming to ASTM C144 gradation. Concrete coarse sand is too sharp and rocky for bed joints, while fine beach sand lacks structural gradation and causes shrinkage cracking.

Contractor Shovel-to-Volume Conversion Rules

Because jobsite mixers rarely measure bulk sand in volumetric cubic-foot boxes, masons calibrate shovel counts using standard #2 contractor shovels:

Standard #2 Square-Point Shovel: A moderately heaped #2 shovel contains approximately 0.22 cubic feet of damp loose sand. This equates to 4.5 shovels per 1.0 cubic foot of sand.
1-Sack Type S Mortar Batch (4.5 cu ft sand): Requires exactly 18 shovels of damp loose sand with 1 sack Portland cement and 0.5 bag hydrated lime.
Moisture Bulking Factor: Sand containing 4% to 6% surface moisture bulks in volume by up to 25% compared to bone-dry sand. ASTM C270 volumetric proportioning accounts for this damp loose jobsite condition.
Round-Point Shovels: Round-point shovels carry 10% to 15% less sand per scoop than square-point spades. If using round-point shovels, increase count from 18 to 20 shovels per 1-sack Type S batch.

Illustrative Material Planning Costs

Planning cost benchmarks for site-mixed raw ingredients across US building material suppliers. Delivery, pallet deposits, and local taxes are excluded:

Portland Cement (Type I/II)$13.50 – $18.00per 94-lb sack

Hydrated Lime (Type S)$11.00 – $15.50per 50-lb bag

ASTM C144 Masonry Sand$45.00 – $65.00per bulk ton delivered

Pricing figures are illustrative planning benchmarks only. Always verify current prices with a dated local supplier quote before placing jobsite purchase orders.

Calculation Assumptions & References Planning Reference

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

Content date: October 2026

Related references: ASTM C270 (Standard Specification for Mortar for Unit Masonry), ASTM C207 (Hydrated Lime for Masonry Purposes - Type S), ASTM C150 / C150M (Portland Cement Specifications), ASTM C144 (Aggregate for Masonry Mortar), TMS 402/602 (Building Code Requirements and Specification for Masonry Structures). Confirm the applicable edition and local requirements.

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

Masonry Q&A

Portland & Lime Mortar Proportions FAQ

Expert contractor answers on ASTM C270 batch ratios, lime types, shovel counts, and CMU block wall mortar requirements.

For load-bearing and reinforced CMU concrete block walls, ASTM C270 specifies Type S mortar with a volumetric ratio of 1 part Portland cement to 1/2 part Type S hydrated lime and 4.0 to 4.5 parts damp loose masonry sand. This delivers a minimum 28-day compressive strength of 1,800 psi while preserving high flexural tensile bond and water retention along CMU face shells.