My helper mixed a big batch of mortar at 7 a.m.

No. Do not retemper mortar that has begun to set. Once the hydration reaction starts (initial set), adding water destroys the cement paste structure, drastically reducing bond strength, compressive strength, and durability. This violates ASTM C270 and TMS 602/ACI 530.1 Art. 3.3.B, which prohibit retempering after initial set.

Practical rules:

• Portland cement/lime mortar: Usable life is ~2.5 hours max (less in heat/wind). If it’s stiff at 10:30 (3.5 hrs), it’s dead. Pitch it.
• Masonry cement mortar: Similar window. Stiffening = hydration underway.
• Only exception: If it stiffened solely from water evaporation (rare, usually <1 hr) and is still plastic, you may retemper once within the first 30–60 minutes per ASTM C270 Note X1.2. At 3.5 hours, this does not apply.

Tell your helper: Mix batches small enough to use in 60–90 minutes. In hot/dry/windy conditions, cover the board, use cool water, and mix smaller batches. Wasted mortar is cheaper than a failed wall.

No — you should not add water to stiffened mortar and continue using it.

Once mortar begins to stiffen (typically within 1–2 hours, depending on temperature, humidity, and mix design), it has started the hydration process. Adding water at this stage — known as “retempering” — weakens the bond, reduces compressive strength, and increases shrinkage cracking. This violates ASTM C270 (Standard Specification for Mortar for Unit Masonry) and most building codes (e.g., IBC Section 2103.2, IRC R606.2.1), which prohibit retempering mortar after initial set.

At 7 a.m. to 10:30 a.m. is 3.5 hours — well beyond the usable life of most mortars (typically 90 minutes max under normal conditions). Even if it seems workable after adding water, the chemical integrity is compromised. The mortar may appear to stick, but it will fail prematurely under load or weather exposure.

What to do instead:

• Discard the stiffened batch.
• Mix a fresh batch — only what you can use within 90 minutes.
• Use a mortar mixer or hoe and wheelbarrow for small batches; consider pre-blended mortar with extended workability additives if delays are frequent.
• Keep materials shaded and cool; use chilled water in hot weather to slow set.

Bottom line: Safety and durability matter more than saving a batch. If in doubt, throw it out and remix. A weak mortar joint can lead to water infiltration, spalling, or structural failure — far costlier than a little extra time and materials.

Always follow manufacturer specs and local code. If unsure, consult a licensed mason or engineer.